Integrins
Integrins are glycoprotein heterodimers that contain a covalently associated alpha and beta subunit. The integrin subunits are transmembrane proteins possessing an extracellular domain for interacting with an extracellular matrix or cellular component, a transmembrane domain spanning the cell membrane, and a cytoplasmic domain for interacting with one or more skeletal components. To date, there are eighteen known alpha subunits that can combine with eight known beta subunits (Gullberg and Lundgren-Åkerlund, 2002), resulting in at least twenty-four different integrin molecules. Integrins can be grouped into subfamilies depending on which beta subunit they contain or alternatively the grouping can be based upon shared structural features of the alpha chain i.e. those integrins characterised by the presence of an additional region known as the I (inserted)-domain. This group includes nine members and thus represents half of the currently known integrin alpha chains (Velling 1999).
Integrin Alpha10Beta1
Recently we discovered a new collagen-binding integrin heterodimer (Camper et al 1998) that contains a novel alpha chain, designated alpha10. This alpha chain is associated with a beta1 subunit (alpha10beta1) and is a member of the I-domain containing integrins. Currently 4 collagen-binding I-domain containing integrins are known, alpha1beta 1, alpha2beta1, alpha10beta1 and alpha11beta1 (Gullberg and Lundgren-Åkerlund 2002).
Sequence analysis shows that alpha10 has the highest identity with alpha11 (43%) and an identity of 33% with alpha1 and 31% with alpha2.
Expression of Integrin Alpha10Beta1
Integrin alpha10beta1 is mainly expressed on chondrocytes in articular cartilage, in the vertebral column, in trachea and in the cartilage supporting the bronchi (Camper et al 2001). The integrin is also found in specialized fibrous tissues such as the fascia of skeletal muscle and tendon, in the ossification groove of Ranvier and in the aortic and atrioventricular valves of the heart (Camper et al 2001).
Function of Integrin Alpha10Beta1 in Cartilage
Chondrocytes are the only cell type in articular cartilage and are responsible for the coordinated synthesis and turnover of the extracellular matrix (ECM) components of the tissue. The two main components of the ECM, apart from water, are different types of collagen and the large aggregating proteoglycan aggrecan. The integrin alpha10beta1 on the chondrocyte cell surface mediates the binding of collagen to the chondrocyte and, like other integrin-extracellular matrix protein interactions (Heino 2000, Boudreau and Jones 1999, Hering 1999), is likely to be responsible for signalling the dynamic state of the surrounding matrix to the cell. Although collagen type II is likely to be an important ligand for alpha10beta1, it is not a prerequisite for alpha10beta1 expression since alpha10beta1 is also present in tissues that lack collagen type II. This implies that alpha10beta1 in vivo can bind to other important extracellular matrix ligands such as chondroadherin and other collagen types e.g. type I and type VI (Tulla et al. 2001).
Identifying tools for studying the biological role and the structural/functional relationships of this integrin with its various extracellular matrix ligands is therefore of great value. Such tools may be of a diagnostic nature for the detection of the presence of alpha10beta1, or maybe of therapeutic value in blocking or stimulating the activity of alpha10beta1.
Antibodies to Integrin Alpha10
Camper et al. (1998) describes the generation of polyclonal antibodies to the cytoplasmic domain of integrin alpha10beta1. The cytoplasmic domain consists of a 16 amino acid (Armulik 2000) sequence extending from the transmembrane domain. This domain is therefore is an ideal immunogen and production of polyclonal antibodies to this domain by immunisation with a peptide, whose sequence corresponds to a region within the cytoplasmic domain, is therefore a relatively simple, straightforward procedure routinely carried out to produce antibodies. (Harlow and Lane 1988). The polyclonal antibodies of Camper et al. (1998) generated in rabbit are of limited use since they are unable to be used on living cells due to their inability to penetrate cells.
General Structure of Naturally Occurring Antibodies
Naturally occurring antibodies comprise of two heavy chains linked together by disulphide bonds and two light chains, one light chain being linked to each heavy chain by disulphide bonds. Each heavy chain has at one end a variable domain (VH) followed by a number of constant domains. Each light chain has a variable domain (VL) at one end and a constant domain at its other end.
It is the variable domains of each pair of light and heavy chains that are directly involved in binding the antibody to the antigen (Harlow and Lane (1999)). The domains of the natural light and heavy chains have the same general structure and each domain comprises of four framework (Fr) regions, whose sequences are somewhat conserved, connected by three hyper-variable or complementarity determining regions (CDRs).
Monoclonal Antibodies of Non-Human Origin in Therapeutic Applications
Murine-derived monoclonal antibodies may cause an immunogenic response in human patients, reducing their therapeutic applicability. To circumvent this problem, humanised antibodies have therefore been developed in which the murine antigen binding variable domain is coupled to a human constant domain. (Morrison et al (1984), Boulianne et al (1984), Neuberger et al (1985)).
In a further effort to resolve antigen-binding functions of antibodies and to minimise the use of heterologous sequences in human antibodies, the CDRs or CDR sequences of murine antibodies are grafted onto the human variable region framework (Jones et al 1986, Riechmann et al 1988, Verhoeyen et al 1988). The therapeutic efficacy of this approach has been demonstrated previously (Reichmann et al (1988) and Hale et al (1989)).
Monoclonal Antibodies in Joint Diseases
Mature articular cartilage has no blood vessels, it is not innervated and normal mechanisms of tissue repair, involving the recruitment of cells to the site of damage does not occur. This means that cartilage has a very poor reparative response to injury and its irreparable breakdown is a common feature of degenerative joint diseases. Repair of such injuries has focused upon different tissue engineering strategies that involve the delivery or in situ mobilisation of cells capable of restoring the pathologically altered architecture and function of the tissue. Tissue engineering approaches for cartilage currently use isolated autologous cells derived from biopsies from healthy sites within the cartilage (autologous chondrocyte transplantation—ACT) (Brittberg 1999). Critical to ACT is the quality of the cells that are implanted back into the joint i.e. the cells should be chondrocytes capable of producing a hyaline-like cartilage (Jobanputra et al 2001).
An alternative strategy to the use of autologous chondrocytes is the use of stem cells with a chondrogenic differentiation capacity such as mesenchymal stem cells (FIG. 1) that can be used in vivo to repair or generate new cartilage (Jorgensen et al 2001, Johnstone and Yoo 2001). Whilst it is well documented that MSCs have the inherent potential to differentiate into osteogenic, chondrogenic, adipogenic and myocardiac cell lineages, there is currently no means of identifying the progenitor cell that will lead to these different lineages. Markers exist to indicate whether the cell is capable of expressing a cartilage phenotype i.e. collagen II and aggrecan, but these proteins are expressed extracellularly after synthesis, and cannot be used for isolation of a chondrogenic cell type.
Antibodies against extracellular integrin epitopes, in contrast to intracellular integrin epitopes, are in general difficult to generate due to a low or absent immunogenic capacity. Normally, this problem is solved by the skilled artisan by administering an adjuvant in parallel with the antigen of interest. Different adjuvants exist and by using one or another, or a combination thereof, a more or less general activation of the host's immune system is generated. Still, as of today's date and with the known accumulated knowledge of adjuvants, no monoclonal antibodies against the extracellular parts of integrin alpha10beta1 have been generated. Thus, an antibody useful in therapy, diagnosis and in situ studies of joint diseases is currently lacking due to the difficulty identified in generating such antibodies.
The one distinguishable feature common to the primary collagen binding integrins receptors is the existence of an I (“inserted”) domain at the N-terminal of the alpha subunit. Only four collagen-binding integrins exist that contain an I-domain (integrin alpha1beta1, alpha2beta1, alpha10beta1 and alpha 11beta1). The I-domains still only show an overall identity of maximum of 60%. The I-domain of the integrin alpha10 is of particular interest since this domain contains unique structural differences compared to the I-domains of the other collagen-binding integrins. These differences include the number of cysteine residues, the high degree of positive amino acids and the recognition of distinct collagen subtypes (Gullberg and Lundgren-Åkerlund 2002, Tulla et al 2001). The I-domain thus comprises a unique ligand binding part and it is thus highly desirable to generate monoclonal antibodies against the I-domain of integrin alpha10, and integrin alpha10beta1.
It is further highly desirable to provide a tool that could identify and select cells of a chondrogenic nature for treatment purposes, in particular for the isolation of chondrocytes, mesenchymal progenitor cells and embryonic stem cells for tissue engineering of cartilage.
It is further highly desirable in the light of aforementioned problems to develop means and methods for identifying diagnostic and therapeutic tools in studying the biological role and the structural/functional relationships of the integrin alpha10beta1 with its various extracellular matrix ligands. Further, there is an unmet need to identifying blocking or neutralizing and stimulatory agents, particularly for chondrocytes, mesenchymal stem cells and other cells expressing the integrin alpha10beta1. In this respect, the present invention addresses these needs and interest.